Various electrical connector devices include a plurality of elongated conductors which electrically interconnect respective inputs and outputs of the connector device. As is known, when an elongated conductor is adjacent to or relatively near another elongated conductor, crosstalk typically is experienced. Crosstalk is defined as the undesirable coupling or the transmission of an electrical signal from one circuit to another nearby circuit. Crosstalk mechanisms are inductive (magnetic field) coupling and by capacitive (electric field) coupling. The level of crosstalk is increased between conductors which are generally parallel, such as the elongated conductors of a cable, modular jacks and plugs, or a printed circuit board.
Crosstalk is undesirable because the integrity and definition of the signals transmitted by the conductors is degraded by the interfering coupled signals. Crosstalk becomes more of a problem with relatively high frequency content signals.
Modular jacks and plugs have certain terminal arrangements to provide standards for intermatability. The plug is normally terminated to a cable having a plurality of parallel conductors which may be connected to a telephone handset or other communications device. The conductors are paired and each pair forms a signal loop or differential signal. Each pair in the cable normally consists of two adjacent twisted conductors. This arrangement in the cable results in certain electrical characteristics of the cable including its characteristic impedance and propagation velocity.
Each jack includes a plurality of elongated terminals which are closely spaced and parallel to one another. A typical jack has eight adjacent and parallel terminals. These terminals are arranged in signal pairs where each pair forms a separate communication circuit. When an electrical signal with a given frequency content is applied to a pair of conductors, an unequal portion of signal energy is transmitted to the individual conductors of an adjacent pair by each conductor of the signal pair. This transmission is primarily due to the capacitive and inductive couplings between adjacent conductors resulting in crosstalk. The extent of the crosstalk is governed by such parameters as the space between the adjacent conductors, the dielectric constant of the material between such conductors, the distance in which such conductors are closely spaced and parallel to one another, and the frequency content of the signal. With modular plugs and jacks being utilized more and more in high frequency applications and with the miniaturization of the plugs and jacks resulting in a very close spacing of the terminals, crosstalk has become a greater problem.
It has been found that crosstalk can be reduced to a great extent through cancellation by the placement of conductors, or the placement of traces on a printed circuit board connecting conductors, within the jack or plug, so as to send signals of an opposite phase against those creating the crosstalk. To decrease crosstalk, the conductors or circuit traces that form both pairs should be routed in a pattern that is opposite in polarity to the pattern that produced the crosstalk. Thus, a signal creating crosstalk with a given polarity is cancelled by a signal of an opposite polarity created by the appropriately placed conductors or traces on the printed circuit board.
The crosstalk reduction techniques used in the past, which include primarily tuned capacitive coupling, have had a desirable effect on the reduction of crosstalk. However, the rerouting of the conductors or circuit traces on the circuit board to create the tuned capacitances contributes to an impedance mismatch of the entire transmission system, including the cable, connector, and circuit board, thereby negatively affecting return loss, voltage standing wave ratio, and combined system attenuation. The use of tuned capacitive coupling alone to cancel both capacitive and inductive crosstalk is inherently unstable. The present invention is directed to solving the various problems outlined above by partial cancellation of pair-to-pair interference or crosstalk created in elongated parallel conductors, while maintaining proper characteristic impedance and longitudinal balance within the electrical signal transmission system. This is accomplished by providing both capacitive and inductive coupling mechanisms in a symmetrical manner to reduce crosstalk. Impedance can be calculated by the using formula of the square root of inductance divided by capacitance. With both inductive and capacitive coupling being tuned in the subject invention, impedance can also be controlled since both the numerator and denominator of the impedance ratio can be adjusted. This is contrasted with the prior art, where primarily tuned capacitive coupling is used to control impedance. With only capacitive coupling being tuned, only the denominator in the impedance ratio can be adjusted.
Inductive coupling is increased by increasing self inductance within a signal pair and mutual inductance between signal pairs. Self inductance is increased by locating the legs of one circuit pair farther away from each other. Because the problem of crosstalk is the greatest with the two inner circuit pairs, their legs are located a distance further away from each other than the distance between the legs of the outer circuit pairs. Mutual inductance is increased when the legs of one circuit pair overlie the legs of another circuit pair. If not overlying each other, then mutual inductance is greater when the center lines of each circuit pair are closer to each other. With both mutual and self inductance increased, the total inductive coupling will be increased. Thus an important feature of this invention is the greater separation of the elongated sections of the conductors forming the inner circuit pairs which increases inductive coupling.
One of the advantages with dual tuned inductive and capacitive coupling is that space can be created between the circuits for corrective capacitive coupling which can be used to further control impedance mismatch. To further adjust the impedance of the circuit, three overlapped plates were designed which extend by way of a tab from each leg of one inner circuit pair. Although one pair of overlapping plates may be adequate, three tabs and three plates are used in the preferred embodiment because three plates are easier to manufacture and spread the capacitance over three areas which will improve the electrical characteristics at higher frequencies. The overlapped plates will create just enough additional capacitance to reduce the impedance caused by the additional inductance. This will result in an improved return loss.